1. Field of the Invention
The present invention generally relates to bone fasteners. More particularly, embodiments disclosed herein relate to coaxially lockable poly-axial bone fastener assemblies.
2. Description of Related Art
The human spine consists of segments known as vertebrae linked by intervertebral disks and held together by ligaments. There are 24 movable vertebrae—7 cervical (neck) vertebrae, 12 thoracic (chest) vertebrae, and 5 lumbar (back) veritebrae. Each vertebra has a somewhat cylindrical bony body (centrum), a number of winglike projections (procsses), and a bony arch. The arches are positioned so that the space they enclose forms the vertebral canal. The vertebral canal houses and protects the spinal cord, and within it the spinal fluid circulates. Ligaments and muscles are attached to various projections of the vertebrae. The bodies of the vertebrae form the supporting column of the skeleton. Fused vertebra make up the sacrum and coccyx, the very bottom of the vertebral column.
The spine is subject to abnormal curvature, injury, infections, tumor formation, arthritic disorders, and puncture or slippage of the cartilage disks. Degeneration caused by trauma, disease, and/or aging may destabilize a portion of the spine and affect surrounding structures. For example, a natural spacing between adjacent vertebrae may be altered due to the destabilization of the spine. Alteration of a natural spacing between adjacent vertebrae may subject nerves that pass between vertebral bodies to pressure. Pressure applied to the nerves may cause pain and/or nerve damage. Maintaining the natural spacing between vertebrae may reduce pressure applied to nerves that pass between vertebral bodies. A spinal stabilization procedure may be used to maintain the natural spacing between vertebrae and promote spinal stability.
Spinal stabilization may involve accessing a portion of the spine through soft tissue. Conventional stabilization systems may require a large incision and/or multiple incisions in the soft tissue to provide access to a portion of the spine to be stabilized. Conventional procedures may result in trauma to the soft tissue, for example, due to muscle stripping.
Spinal stabilization systems for a lumbar region of the spine may be inserted during a spinal stabilization procedure using a posterior spinal approach. Conventional systems and methods for posterolateral spinal fusion may involve dissecting and retracting soft tissue proximate the surgical site. Dissection and retraction of soft tissue may cause trauma to the soft tissue, and extend recovery time. Minimally invasive procedures and systems may reduce recovery time as well as trauma to the soft tissue surrounding a stabilization site.
U.S. Pat. No. 6,530,929 to Justis et al. (hereinafter “Justis”), which is incorporated herein by reference, describes minimally invasive techniques and instruments for stabilizing a bony structure in an animal subject. Justis provides a method for using an instrument to connect at least two bone anchors with a connecting element. The instrument is secured to the anchors and manipulated to place the connecting element in a position more proximate the anchors.
In some spinal stabilization systems, pedicle screws can be used as bone anchors. In a traditional poly-axial pedicle screw, a spherical pocket or recess in the head of the screw is used to allow the shank of the screw to rotate in multiple directions about the spherical recess within the head of the screw.
U.S. Pat. No. 6,716,214 to Jackson (hereinafter “Jackson”), which is incorporated herein by reference, describes a poly-axial bone screw having a bone implantable shank, a head, and a retaining ring. The ring and the shank connect to form a ball and socket joint with the head and allow free rotation to a selected angular configuration. The ring has a restrictive neck and the angle of rotation of the ball and socket joint is only restricted by engagement of the shank neck with the ring restrictive neck on the head.
U.S. Patent Application Publication No. 2008/0097457 by Warnick (hereinafter “Warnick”), which is incorporated herein by reference, describes a pedicle screw system having a tulip assembly. Before a rod is placed in the tulip assembly, the tulip assembly may be locked onto a pedicle screw via a poly-axial lock. The poly-axial lock allows the tulip assembly to move poly-axially in relation to the screw.
U.S. Patent Application Publication No. 2008/0140135 by Konieczynski et al. (hereinafter “Konieczynski”), which is incorporated herein by reference, describes a poly-axial fixation device having a bone screw with a spherical head. A snap ring is utilized to frictionally engage the spherical head. The frictional forces created by the snap ring act on the spherical head to allow the bone screw to be set at a desired angular orientation with respect to a receiver member. The frictional forces can be overcome by grasping and moving the bone screw with respect to the receiver member to change the angular orientation.